Universal Time
Published on Aug 25, 2006 at 6:51 pm.
1 Comment.
Filed under time.
The last Full Moon was August 23, at 19:10 UT. The First Quarter Moon will be August 31, at 22:56 UT. What’s this UT stuff?
UT stands for Universal Time. So, what does that mean? Well, you see there are certain things that happen at times local to where you are. For example, the time that the Sun is highest in the sky we generally call local noon, and it occurs at a different time for every longitude on Earth. But, other events happen at very specific times. For example, the moment that the Moon is most directly between the Sun and Earth is the time of the New Moon. It happens at a particular moment. But, this is a different time of day for most people, depending upon where they live. So, naturally, it makes sense to have some time that we can all agree upon. Here in the United States, network television programs are broadcast at specific times. They advertise the times as something like “tonight at 8, 7 Central.” The assumption is that people will know that that is 8 o’clock Eastern and 7 o’clock Central time. But, it doesn’t make sense to give the time of the New Moon with 24 different time zones, so we pick one.
By convention, we pick the time at the Prime Meridian. All you have to do is to know your local offset from that time. But, it isn’t quite that simple. How do we define time?
I made reference to noon earlier. What does that mean? Most people think of noon as the time that the Sun is directly overhead. That is what I was taught in elementary school. Being a budding scientist, I stuck a stick in the ground, and I figured out very quickly that something was wrong. If the Sun were directly overhead, then it should not cast a shadow. When the clock struck noon, there was still a shadow. Worse, it wasn’t even at its shortest at noon! So, what was up?
Well, the problem was that the term “noon” used to mean the time that the Sun was highest in the sky. The line running between north and south is the meridian, so the highest that a celestial object can be is when it is on the meridian. When the Sun is before the meridian, it is ante-meridian (AM), and when it is after the meridian, it is post meridian (PM). But, as my experiment showed, that isn’t the way that it is anymore. The problem is twofold. First, the Earth has an elliptical orbit that speeds up and slows down. That makes the length of the day vary by a few seconds. But, it is difficult and confusing to make a clock that runs at a different rate every day of the year. So, we take an average. Clocks are designed to run through twenty four hours on the average length of the day. As I said, the actual day is off by a few seconds. But enough days a few seconds off eventually makes the clock time and the time according to the Sun off by several minutes — as much as about a quarter hour. So, we can define two types of time: Local Solar Time (LST) and Local Mean Time (LMT). The Local Solar Time, or sometimes called Sundial Time, is the time according to the Sun. Local Mean Time is the time according to the average solar motion. LMT varies back and forth from LST by several minutes, but on four days of the year it matches. What days those are slowly shift.
But, there is another reason that the clock noon and the local solar noon don’t coincide. The local time is determined by the position of the observer on the surface of the Earth. Each longitude will have the Sun on its meridian at a different time. So, that means that everyone would have a different time. Worse, if you moved just a little east or west, you’d have to adjust the time by several seconds or minutes. So, for convenience, we often define a time so that all the clocks in a certain geographic region are set to the LMT at a particular longitude. The Central Time Zone in the United States is supposed to be the local mean time at 90 degrees west longitude. Eastern Time is the LMT at 75 degrees west longitude.
Then, to make things more confusing, during part of the year, we have a sneaky trick to get people to wake up earlier. We just call it later. So, instead of calling it 7am, we call it 8am. This is Daylight Saving Time here in the US. So, the actual solar noon often occurs at 1:30pm here in Texas when we are on Daylight Saving Time. Currently, Daylight Saving Time starts the first Sunday in April and lasts until the last Sunday in October. Next year, 2007, that changes, though. In 2007, Daylight Saving Time will commence at 2am on the second Sunday in March and last until 2am on the first Sunday in November. Hmm. I guess all those computers running Windows that automatically reset the clock will be off unless the latest updates have been downloaded.
So, we want our reference time to be based upon the local mean time at the prime meridian. Most people call this Greenwich Mean Time, or Zulu Time. However, physicists and astronomers have to be different. We call this Universal Time. or UT.
Well, as often happens, there are still complications. You see, Earth does not rotate at a constant rate. It slightly speeds up or slows down due to tectonic activity. As large masses move around in the Earth, conservation of angular momentum causes the slight change in rotation rate. Also, tidal forces between the Earth and the Moon are slowing the rotation of the Earth. Taking account of these factors, that means that Universal Time has to be continually shifting to compensate for these effects. Remember, UT noon is the average noon of the year, and if you change the rotation of the Earth, you change when noon will occur. But, it would be a pain to keep adjusting clocks every day by a few milliseconds. So, we wait until the adjustment adds up to a full second. Clocks keep going at a constant rate, and if needed then we add or take away a second on June 30 or December 31 to shift the clocks to be closer to being in sync with the actual UT. This clock time that is within a second of UT we call Coordinated Universal Time, or UTC. Often, when astronomers refer to Universal Time, we really mean UTC, but are just too lazy to say the whole thing. The two times run within a second of each other though, so it is usually good enough, and if you need more precision, then you specify exactly which UT you mean.
Government agencies keep track of the official time and comparisons to UTC. So, how do you know what the official time is? Well, if you live in the US, you can listen to WWV radio on shortwave at 5, 10, and 15 MHz. But, most people don’t have shortwave radios. Worse, most of the shortwave receivers that I see sold at the stores around here don’t have those frequencies available! But, that’s OK if you have a computer. Since you are reading this, I assume that you have access to a computer. The US National Institute of Standards and Technology website has links to the official time. You can get the time directly, or you can go to their internet time service link and download a program that will periodically reset your computer clock to the official legal time in the US.
Then, you’ll know what time it is!
-Astroprof
Seth Mantel Clocks on June 24, 2008 at 8:41 am: 1
So that’s why its called AM PM! Very informative post. I can’t believe I was interested to read it till the end.